1. Field of the Invention
This invention relates to the corrosion protection of pipes, vessels and other corrodible substrates.
2. Introduction to the Invention
It is well known to protect substrates from corrosion by establishing a corrosion-protecting potential difference between the substrate and a counter-electrode. In impressed current systems, which are generally preferred, a DC power source is used to establish (or help to establish) the desired potential difference, and the anode is preferably composed of a material which is resistant to corrosion, e.g. platinum, graphite, or a conductive polymer. Generally, the substrate is the cathode, but there are systems in which the substrate is the anode and the protection is provided by an oxide layer which is built up and maintained on the substrate. In sacrificial systems, the counter-electrode is an anode composed of a metal which is less noble than the substrate, and the desired potential difference is established automatically. Reference may be made for example to U.S. Pat. Nos. 4,502,929 (Steward et al), 4,473,450 (Nayak et al), 4,319,854 (Marzocchi), 4,255,241 (Kroon), 4,267,029 (Massarsky), 3,868,313 (Gay), 3,798,142 (Evans), 3,391,072 (Pearson), 3,354,063 (Shutt), 3,022,242 (Anderson), 2,053,314 (Brown) and 1,842,541 (Cumberland), U.K. Patents No. 1,394,292 and 2,046,789A, and Japanese Patents Nos. 35293/1973 and 48948/1978, as well as copending commonly assigned application Ser. No. 684,900 (Stratfull), the disclosures of which are incorporated herein by reference.
The known corrosion systems suffer from serious disadvantages in a number of commercially important situations. One disadvantage is a failure to obtain sufficiently uniform current distribution on the substrate. This disadvantage can arise from the use of one or more discrete electrodes; or from the use of a distributed electrode, e.g. a platinum wire, whose radial resistance to the substrate is low, so that at high currents the current density on the anode decreases rapidly as the distance from the power source increases; and/or because the substrate is shielded (including those situations in which the substrate has a complex shape which results in one part of the substrate being shielded by another part of the substrate). The flexible elongate anodes disclosed in U.S. Pat. Nos. 4,502,929 and 4,473,450, which comprise a low resistance core surrounded by a conductive polymer coating, are very useful in mitigating this disadvantage, but they cannot be used at the high current densities which are required in certain situations, for example the protection of structures which have no protective coating thereon. Another disadvantage is the relatively short life of anodes (including the electrical connections thereto), especially when exposed to environments which are highly corrosive or which contain oily contaminants, and in the case of platinum anodes, when exposed to fresh potable water, and the difficulty and expense of repairing or replacing the anodes when this becomes necessary.